When need precisely to align one object to another object, usual way is to put align mark to each object, then to measure the shift between these two marks to decide the shiftiness of the two object, then to align them. Unfortunately, when the precision requirement reaches nm level, it could not be accurately measured optically because optical wavelets limitation. It can use light diffraction such as applying grating structure, but because the line thickness also affect the light diffraction, to use grating diffracting to measure layer to layer shiftiness still has problems.